In this chapter from Core Java for the Impatient, you will learn about interfaces and lambda expressions, including static and default methods, method and constructor references, processing lambda expressions, variable scope, higher-order functions, and local inner classes.

Java was designed as an object-oriented programming language in the 1990s when object-oriented programming was the principal paradigm for software development. Interfaces are a key feature of object-oriented programming: They let you specify what should be done, without having to provide an implementation.

Long before there was object-oriented programming, there were functional programming languages, such as Lisp, in which functions and not objects are the primary structuring mechanism. Recently, functional programming has risen in importance because it is well suited for concurrent and event-driven (or “reactive”) programming. Java supports function expressions that provide a convenient bridge between object-oriented and functional programming. In this chapter, you will learn about interfaces and lambda expressions.

The key points of this chapter are:

An interface specifies a set of methods that an implementing class must provide.

An interface is a supertype of any class that implements it. Therefore, one can assign instances of the class to variables of the interface type.

An interface can contain static methods. All variables of an interface are automatically static and final.

An interface can contain default methods that an implementing class can inherit or override.

The Comparable and Comparator interfaces are used for comparing objects.

A lambda expression denotes a block of code that can be executed at a later point in time.

Lambda expressions are converted to functional interfaces.

Method and constructor references refer to methods or constructors without invoking them.

Lambda expressions and local inner classes can access effectively final variables from the enclosing scope.

3.1 Interfaces

An interface is a mechanism for spelling out a contract between two parties: the supplier of a service and the classes that want their objects to be usable with the service. In the following sections, you will see how to define and use interfaces in Java.

3.1.1 Declaring an Interface

Consider a service that works on sequences of integers, reporting the average of the first n values:

public static double average(IntSequence seq, int n)

Such sequences can take many forms. Here are some examples:

A sequence of integers supplied by a user

A sequence of random integers

The sequence of prime numbers

The sequence of elements in an integer array

The sequence of code points in a string

The sequence of digits in a number

We want to implement a single mechanism for deal with all these kinds of sequences.

First, let us spell out what is common between integer sequences. At a minimum, one needs two methods for working with a sequence:

Test whether there is a next element

Get the next element

To declare an interface, you provide the method headers, like this:

public interface IntSequence {
boolean hasNext();
int next();
}

You need not implement these methods, but you can provide default implementations if you like—see Section 3.2.2, “Default Methods,” on p. 100. If no implementation is provided, we say that the method is abstract.

NOTE

All methods of an interface are automatically public. Therefore, it is not necessary to declare hasNext and next as public. Some programmers do it anyway for greater clarity.

There are infinitely many squares, and an object of this class delivers them all, one at a time.

The implements keyword indicates that the SquareSequence class intends to conform to the IntSequence interface.

CAUTION

The implementing class must declare the methods of the interface as public. Otherwise, they would default to package access. Since the interface requires public access, the compiler would report an error.

The SquareSequence and DigitSequence classes implement all methods of the IntSequence interface. If a class only implements some of the methods, then it must be declared with the abstract modifier. See Chapter 4 for more information on abstract classes.

3.1.3 Converting to an Interface Type

This code fragment computes the average of the digit sequence values:

IntSequence digits = new DigitSequence(1729);
double avg = average(digits, 100);
// Will only look at the first four sequence values

Look at the digits variable. Its type is IntSequence, not DigitSequence. A variable of type IntSequence refers to an object of some class that implements the IntSequence interface. You can always assign an object to a variable whose type is an implemented interface, or pass it to a method expecting such an interface.

Here is a bit of useful terminology. A type S is a supertype of the type T (the subtype) when any value of the subtype can be assigned to a variable of the supertype without a conversion. For example, the IntSequence interface is a supertype of the DigitSequence class.

NOTE

Even though it is possible to declare variables of an interface type, you can never have an object whose type is an interface. All objects are instances of classes.

3.1.4 Casts and the instanceof Operator

Occasionally, you need the opposite conversion—from a supertype to a subtype. Then you use a cast. For example, if you happen to know that the object stored in an IntSequence is actually a DigitSequence, you can convert the type like this:

You can refer to them by their qualified name, SwingConstants.NORTH. If your class chooses to implement the SwingConstants interface, you can drop the SwingConstants qualifier and simply write NORTH. However, this is not a common idiom. It is far better to use enumerations for a set of constants; see Chapter 4.

NOTE

You cannot have instance variables in an interface. An interface specifies behavior, not object state.